1. Field of the Invention
The present invention relates to chemical mechanical polishing of an organic film such as a resist film, and a method of manufacturing a semiconductor device.
2. Description of the Related Art
In the process of manufacturing a semiconductor device, a photoresist film is employed as an expendable film for fabricating a desired structure. For example, after trenches have been formed in a semiconductor substrate or after holes have been formed in an insulating film, a photoresist is coated to form an expendable film. Then, the expendable film is subjected to a recessing process or a peeling process to obtain a desired structure. This kind of process has been employed for forming a buried strap for electrically connecting a storage node electrode with a diffusion layer of a cell transistor in the manufacture of a semiconductor memory provided with a trench capacitor. Further, this kind of process is also useful in a process of preliminarily forming a via-hole (via first DD) in the process of forming a Cu dual damascene structure (Cu DD structure).
In either of the cases, the film thickness of the photoresist film is required to be uniform throughout the entire surface of wafer. However, when a resist is applied to a region where trenches and holes are formed in a pattern of high density so as to bury these trenches and holes with the resist, the volume of the resultant resist film diminishes at such a region of high pattern density. Therefore, there will be generated a large magnitude of non-uniformity in thickness, e.g., of the order of several hundreds of nanometers, between the film thickness of the resist film located over a region of high pattern density and the film thickness of the resist film located over a region of low pattern density or over a field region.
This non-uniformity in film thickness of the resist film will be further enlarged in a subsequent recessing, thereby degrading the configuration of device. Additionally, this non-uniformity in film thickness of the resist film would become a cause for decreasing the depth of focus or for reducing device yield.
With a view to overcoming these problems resulting from the aforementioned non-uniformity in film thickness of the resist film, there has been proposed to a method for flattening the photoresist by chemical mechanical polishing (CMP) after finishing the coating of the resist. However, the application of CMP to a photoresist film has been found to have the following problems.
Namely, conventional CMP for polishing a photoresist film is accompanied with problems that it takes a relatively long time for polishing the photoresist film and that the stability in terms of polishing time is poor. These problems will become more conspicuous especially when a resist film of large thickness, e.g., 3 μm, is subjected to CMP. More specifically, the polishing of such a resist film will take as long as 200 to 270 seconds and stability in terms of polishing time will also be poor.
Moreover, it will be difficult to suppress the dishing, and the surface of wafer that has been polished will be poor in uniformity. If the in-plane uniformity of the wafer is poor, it will become invite non-uniformity in depth of the recesses to be subsequently formed, thereby making it difficult to obtain a uniform and desired in-plane configuration of the wafer. For example, the existence of non-uniformity in depth of the recesses on forming a buried strap will become a direct cause for non-uniformity of collar oxide film, resulting in non-uniformity in electrical resistance of the buried strap.
On polishing a resist film whose hardness has been increased through the baking thereof at a high temperature, inorganic particles such as silica and alumina are employed as an abrasive grain to secure a practical process margin and productivity. In this case, the inorganic particles tend to remain in the trenches, thereby raising problems that the surface condition of the wafer is degraded and that the residual inorganic particle will become a mask in the recessing step subsequent to the CMP step.